I have the "Letters of C.S. Lewis" edited by W.H.Lewis first printed in 1966 by Bles.

This is not the same book that you mention, but this book has a couple of problems that relate to your enquiry. It has no index, so it is difficult to find all Tolkien references and it also appears to not have a single letter addressed directly to Tolkien.

The "Collected Letters" edited by Walter Hooper are in three volumes and it looks like volume two may contain some letters sent to Tolkien.